In conventional semiconductor devices sealed with a molding resin, the miniaturization of semiconductor elements has led to the problem of volatilization insufficiency or malfunctions, which problem is caused by the stress of the sealing resin.
To overcome this problem, a technique of covering an element with a stress-buffering film consisting of a resin layer has been proposed. For example, the semiconductor device disclosed in JP-A-55-50645 employs a surface-protective film made of a silicone ladder resin having in side chains both a group which functions to enhance heat resistance, e.g., phenyl or methyl, and a group which forms a crosslink upon heating, e.g., an alkyl group. (The term "JP-A" as used herein means as "unexamined published Japanese patent application.")
FIG. 3 shows sectional views illustrating steps of a process for producing the conventional semiconductor device disclosed in the above reference.
The process for producing this conventional semiconductor device is explained below.
First, a base region 32 and an emitter region 33 are formed on a semiconductor substrate 31 made of silicon, as shown in FIG. 3 (a).
A cyclohexanone solution of a synthetic silicone resin obtained by polymerizing phenyltriethoxysilane with heating is then applied on the resulting structure with a spinner, and the coating is dried to form a silicone ladder resin film 34 as shown in FIG. 3 (b).
Subsequently, a photoresist pattern 35 is formed thereon which has openings for the electrode-mounting part of the semiconductor substrate 31 which serves as a collector region and for the electrode-mounting parts of the base region 32 and the emitter region 33, as shown in FIG. 3 (c).
The silicone ladder resin film 34 is then etched with 1,1,1-trichloroethane using the photoresist pattern 35 as a mask. Thereafter, the photoresist 35 is removed and the resulting structure is heated at 350.degree. C. for 1 hour to form a crosslinked silicone ladder resin pattern 34a as shown in FIG. 3 (d).
Subsequently, a deposited aluminum film 36 is formed so as to fill the openings of the silicone ladder resin pattern 34a as shown in FIG. 3 (e), and a photoresist pattern 35a having a given shape is formed thereon as shown in the figure.
The deposited aluminum film 36 is then etched using the photoresist pattern 35a as a mask to thereby form aluminum electrodes 36a as shown in FIG. 3 (f). Thus, the desired semiconductor device is fabricated.
The MOS transistor according to the above-described conventional technique, which has a film of a silicone ladder resin such as that obtained by polymerizing phenyltriethoxysilane with heating, has proved to have a stable leakage current between the source and drain even after standing in a high-temperature atmosphere.
On the other hand, FIG. 4 is a sectional view illustrating the semiconductor device disclosed in JP-A-56-118334. In the figure, numeral 41 denotes a semiconductor substrate and 44's denote aluminum wiring formed on the semiconductor substrate 41 and an aluminum film serving as an aluminum electrode pad for wire bonding. Further, numeral 45 denotes a PSG film and 46 denotes a surface-protective film made of a polyimide. A feature of this semiconductor device resides in that due to the surface-protective film 46, the device is free from soft errors caused by .alpha.-rays, and the PSG film 45 as an undercoat does not suffer cracking even when it has mechanical stress.
In the above-described reference, an alkali solution is used in the etching of the polyimide film for forming the surface-protective layer 46. This etchant, however, corrodes the aluminum surface, which corrosion may result in an aluminum pad having a rough surface. When an Au wire is bonded to this surface-roughened aluminum pad, the surface roughness of the pad results in a defective connection between the aluminum pad and the Au wire, leading to decreased reliability of the semiconductor device.
The surface of the aluminum pad can be prevented from being roughened due to the presence of the PSG film 45. That is, prior to the formation of the polyimide film in fabricating this semiconductor device, the PSG film 45 is formed by the CVD method and the polyimide film formed thereafter is etched to provide openings in the polyimide film. The PSG film 45 disposed over the aluminum pad is thereafter selectively removed by etching with a mixture of HF and NH.sub.4 F.
Because of the factors described above, however, the conventional semiconductor devices have a problem that when the devices are packaged, the stress caused by the molding resin cannot be buffered.
This is because the attainable thickness of the silicone ladder resin film in one of the conventional devices as described in the aforesaid JP-A-55-50645 is up to 2 .mu.m, so that this film is too thin to function as a stress-buffering film. Such thickness limitation of the silicone ladder resin is attributable to the molecular weight thereof which is not so high.
Hitherto, a silicone ladder resin having, in side chains, both a group which enhances heat resistance (e.g., phenyl or methyl) and a group which forms a crosslink upon heating (e.g., an alkyl group), or a polyimide resin have been employed for the surface-protective film. The higher the heat resistance of the surface-protective film of a semiconductor device, the higher the reliability of the device. The heat resistance of silicone ladder resins varies depending on the kind of the side chains thereof. In the air, the phenylated silicone ladder resins have the highest heat resistance of from 500.degree. to 550.degree. C., and are superior in heat resistance to polyimide resins. In an oxygen-free atmosphere, e.g., in nitrogen, however, the methylated silicone ladder resins have a heat resistance as high as 700.degree. C. In view of the fact that those semiconductor devices are used in such a state that the surface-protective film is sealed with a molding resin, the optimal material of the surface-protective film is a methylated silicone ladder resin. However, it has been difficult to obtain a high-molecular silicone ladder resin having methyl groups in side chains without gelation, so that formation of a thick film has been difficult. That is, the thicknesses of the conventional silicone ladder resins are insufficient for buffering the stress caused by the molding resins.
Another problem of the semiconductor devices, as decribed in the aforesaid JP-A-55-50645, is concerned with the use of 1,1,1-trichloroethane for etching. 1,1,1-trichloroethane not only reacts with water to generate hydrogen chloride, which is corrosive, but also readily decomposes upon exposure to a fire to generate gases including hydrogen chloride. Because of such properties of 1,1,1-trichloroethane, care should be taken in handling the same, and etching with this compound is limited in the materials of the etching apparatus and etching vessels.
On the other hand, polyimides as used in the aforesaid JP-A-56-118334 function as a stress-buffering film, but have a problem of inferiority in heat resistance and stability. Use of a polyimide has another drawback in that a PSG film should be formed and patterned in order to prevent the surface of the aluminum pad from being roughened during etching of the polyimide resin film, making the process complicated.
A further problem of the conventional semiconductor devices is that when the surface-protective film is formed on a UV-ray erasion type semiconductor memory such as an OTPROM, erasion with UV rays is impossible or usable UV rays are limited in wavelength, etc. For example, when a polyimide resin film is employed, UV light is unusable for erasing stored information because polyimide resins do not transmit light having a wavelength of 400 nm or shorter. Use of the silicone ladder resin disclosed in the aforesaid JP-A-55-50645 has a drawback in that the erasion of stored information by UV irradiation is inefficient because of the presence of phenyl groups in side chains of the resin. This is because the silicone ladder resin having phenyl groups has an absorption wavelength region of from 250 to 270 nm, although it transmits light having a wavelength of 290 nm or longer.